The present invention relates to a CDMA receiver, path detection method used therefor, and recording medium which records a path detection control program and, more particularly, to a technique of measuring a delay profile and selecting several paths having high signal powers within the measurement range.
DS-CDMA (Direct Sequence-code Division Multiple Access) is a scheme of allowing a plurality of users to perform communication by using the same frequency band. In this scheme, spreading codes are used to identify the respective users.
In mobile communication, since multiplex reception waves vary in propagation path length, multiplex waves with different propagation delay times interfere with each other. In DS-CDMA communication, since information data is band-spread by a high-rate spreading code having a period shorter than the propagation time, the respective multiplex waves with different propagation delay times can be demultiplexed/extracted.
Since the position of a mobile station changes with respect to a base station, this delay profile (the signal power distribution with respect to delay times) also changes over time. In addition, a signal from each path is subjected to Rayleigh variation in a place with obstacles.
In DS-CDMA communication, a plurality of multipath signals that are temporally separated from each other, have different propagation delay times, and undergo Rayleigh variations are collected and in-phase-synthesized (Rake-synthesized) to obtain a diversity effect, thereby improving reception characteristics. With regard to a predetermined reception quality (bit error rate), the transmission power can be reduced owing to the diversity effect accompanying Rake synthesis. As a consequence, the interference powers with respect to other users inside and outside a single cell decrease. This makes it possible to increase the subscriber capacity in a predetermined frequency band.
As described above, however, since the position of each mobile station relatively changes with respect to the base station, the delay profile also changes, and the delay times of the paths to be subjected Rake synthesis also change. This makes it necessary for a receiver to have multipath search and tracking functions that allow the receiver to follow changes in delay profile under a mobile communication environment and Rake-synthesize a plurality of paths capable of instantaneously obtaining maximum signal powers.
As the CDMA receiver described above, the receiver shown in FIG. 22 is available, which is comprised of an antenna 21, RF reception circuit section 22, A/D (Analog/Digital) conversion section 23, multipath search section (multipath detection means) 24, and Rake synthesis reception section 25 for in-phase-synthesizing (Rake-synthesizing) a plurality of paths.
In the conventional CDMA receiver having such an arrangement, the multipath search section 24 measures a delay profile (a signal power distribution with respect to delay times), selects several paths having high signal powers within the measurement range, and notifies the Rake synthesis reception section 25 of the timings of the paths. The Rake synthesis reception section 25 performs despreading in units of paths on the basis of the timing information, and performs Rake synthesis, thereby obtaining a path diversity effect.
The Rake synthesis reception section 25 may have a means (path tracking means) for following the movement of an independently designated path. In this case, the multipath search section 24 must send path information to the Rake synthesis reception section 25 at least in an initial period or in predetermined cycles. This CDMA receiver and multipath search method are disclosed in Japanese Patent Laid-Open No. 9-181704 or the like.
In the above conventional CDMA receiver, the multipath search section measures a delay profile, selects several paths having high signal powers within the measurement range, and notifies the Rake synthesis reception section of the timings of the paths.
In general, however, a delay profile contains a large quantity of data, and hence it takes time to search for peaks from all the data. For this reason, to search for several paths from the measured delay profile, maximum values corresponding to the number of paths to be searched out must be retrieved, or all the profile data must be sorted.
It therefore takes much time to perform maximum value retrieval by using a DSP (Digital Signal Processor) or the like. In addition, the current consumption increases. When maximum value retrieval is implemented by hardware, the hardware size becomes large, and an optimal means for searching for a required number of correlation peaks from a delay profile is required.
It is an object of the present invention to provide a CDMA receiver, path detection method, and recording medium which records a path detection control program, which can greatly reduce the processing amount in a path search section, reduce current consumption, simplify the hardware arrangement, and improve follow-up characteristics with respect to changes in path.
In order to achieve the above object, according to the present invention, there is provided a CDMA receiver comprising a Rake reception circuit for performing in-phase synthesis of reception signals from a plurality of paths, wherein a delay profile indicating a signal power distribution with respect to delay times of the reception signals is measured, an interference wave power value is estimated on the basis of the measured delay profile, valid data are extracted from the delay profile on the basis of the estimated interference wave power value, a plurality of correlation peak positions are detected from the extracted valid data, and path allocation to the Rake reception circuit is determined on the basis of the correlation peak positions.